Arbitrary breakpoints via custom mixin

respond-to.scss

@mixin respond-to($primary, $secondary:min-width, $nq:false, $nq-class:lt9) {
  // Convert pixel values to ems
  @if unit($primary) == 'px' {
    $primary: pem($primary);
  }
   @if type-of($secondary) == number and unit($secondary) == 'px' {
    $secondary: pem($secondary);
  }

  // Print out Media Queries
  @if $secondary == max-width {
    @media screen and (max-width: $primary - .001) { @content; }
    @if $nq == true {
      .#{$nq-value} &{
        @content;
      }
    }
  }
  @else if $secondary == min-width {
    @media screen and (min-width: $primary) { @content; }
    @if $nq == true {
      .#{$nq-value} &{
        @content;
      }
    }
  }
  @else {
    @media screen and (min-width: $primary) and (max-width: $secondary - .001) { @content; }
    @if $nq == true {
      .#{$nq-value} &{
        @content;
      }
    }
  }
}